Studies of Performance of Cs2TiI6−XBrX (Where x = 0 to 6)-Based Mixed Halide Perovskite Solar Cell with CdS Electron Transport Layer

The present research work represents the numerical study of the device performance of a lead-free Cs2TiI6−XBrX-based mixed halide perovskite solar cell (PSC), where x = 1 to 5. The open circuit voltage (VOC) and short circuit current (JSC) in a generic TCO/electron transport layer (ETL)/absorbing layer/hole transfer layer (HTL) structure are the key parameters for analyzing the device performance. The entire simulation was conducted by a SCAPS-1D (solar cell capacitance simulator- one dimensional) simulator. An alternative FTO/CdS/Cs2TiI6−XBrX/CuSCN/Ag solar cell architecture has been used and resulted in an optimized absorbing layer thickness at 0.5 µm thickness for the Cs2TiBr6, Cs2TiI1Br5, Cs2TiI2Br4, Cs2TiI3Br3 and Cs2TiI4Br2 absorbing materials and at 1.0 µm and 0.4 µm thickness for the Cs2TiI5Br1 and Cs2TiI6 absorbing materials. The device temperature was optimized at 40 °C for the Cs2TiBr6, Cs2TiI1Br5 and Cs2TiI2Br4 absorbing layers and at 20 °C for the Cs2TiI3Br3, Cs2TiI4Br2, Cs2TiI5Br1 and Cs2TiI6 absorbing layers. The defect density was optimized at 1010 (cm−3) for all the active layers.


Introduction
A solar cell is made for the conversion of solar energy into electrical energy directly, which has undergone continuous development from the past few years due to its superior photovoltaic (PV) properties. Initially, 1st generation wafer-based PV technology was developed but the production cost was relatively higher with lower conversion ability. The production cost became lower after the introduction of thin film-based 2nd generation solar cells, but still the power conversion efficiency (PCE) remained low. However, the problems including cost and PCE were nullified after the development of thin film-based 3rd generation PV technology. Perovskite solar cells (PSCs) have a great capability towards photovoltaic applications and are rapidly emerging as 3rd generation solar cells [1,2]. The journey started with Kojima et al. 2009, with halide perovskite ABM 3 (A: organic CH 3 NH 3 + ; B: Pb, M: Br, I), and they recorded 3.8% of power conversion efficiency (PCE, %) [1]. Later, Yang et al. 2015, andYin et al. 2015 further carried out extensive research on material, deposition process, fabrication methods and device structure that enhanced the PCE up to 20.1% experimentally, and theoretically 31.4% [3,4]. Hosseini et al. 2022 showed the effect of non-ideal conditions on lead-based CH 3 NH 3 PbI 3 perovskite material [5]. Despite the higher conversion efficiency in the laboratory, it has several issues in terms of commercialization such as environment protection from the toxic lead (Pb) component, and stability under environment conditions due to the organic component used in the ABM 3 structure.
To address such issues, researchers considered using lead-free inorganic material as the absorbing material for the photovoltaic applications. Titanium (Ti)-based PSCs were introduced by Ju 5.69 mA/cm 2 current density (J SC , mA/cm 2 ) and 56.4% fill factor (FF, %) with Cs 2 TiBr 6 absorbing material [6,7]. Shyma et al. 2022 employed SCAPS-1D-based simulation on Sn-based perovskite material CH 3 NH 3 SnI 3 to investigate the various parameters of the active materials and the selection of the appropriate ETL (electron transport layer) for the device [8]. On the other side, Omarova et al. 2022 showed the selection of the optimal HTL (hole transport layer), and also determined that electrodes can minimize the effects of material defects to improve the device performance [9]. By taking the knowledge from the above discussion, our research article has pinpointed several vital contributions including developing a theoretical model of a lead-free mixed halide FTO/CdS/Cs 2 TiI 6−X Br X /CuSCN/Ag-based structure where all the materials used for the SCAPS simulation are inorganic in nature, and then important physical, opto-electronic parameters such as thickness of material, device temperature and defect density of the material are optimized. For this optimization, we have analyzed the device performance parameter PCE with the variation in thickness of the active materials, followed by the device performance with the device temperature and defect density.

Device Architecture and Simulation
In our proposed FTO/CdS/Cs 2 TiI 6−X Br X /CuSCN/Ag-based planar solar cell device model, the band gap of ETL (electron transport layer) materials, CdS and HTL material CuSCN are taken to be 2.4 eV, 3.26 eV and 3.4 eV, respectively, and the absorbing layer is tuneable under 1.0-1.78 eV [10][11][12][13][14][15]. The working temperature for the simulation is maintained at 27 • C with −0.8 V to 0.8 V bias voltage in the SCAPS-1D simulator. Here, Figure 1 shows the schematic view of the proposed structure. the simulation is carried out with illumination of AM 1.5 with the light power of 1000 W/m 2 under Gaussian energy distribution, and its characteristic energy is set to 0.1 eV. With Br doping the lattice parameter, band structure and the optical properties of Cs 2 TiI 6 can be changed. Thus, due to the doping effect, both Cs 2 TiI 2 Br 4 and Cs 2 TiI 3 Br 3 are suitable for solar cell applications. On the other side, based on the superior optical coincidence index and better absorption coefficient, Cs 2 TiI 2 Br 4 and Cs 2 TiIBr 5 are ideal for light harvesting applications [16]. The details of the device materials' properties and active materials' basic parameters taken for the work are shown in Tables 1 and 2, respectively. The symbols, i.e., E g , denote the band gap energy; 3rd generation PV technology. Perovskite solar cells (PSCs) have a great capability towards photovoltaic applications and are rapidly emerging as 3rd generation solar cells [1,2]. The journey started with Kojima et al. 2009, with halide perovskite ABM3 (A: organic CH3NH3 + ; B: Pb, M: Br, I), and they recorded 3.8% of power conversion efficiency (PCE, %) [1]. Later, Yang et al. 2015, andYin et al. 2015 further carried out extensive research on material, deposition process, fabrication methods and device structure that enhanced the PCE up to 20.1% experimentally, and theoretically 31.4% [3,4]. Hosseini et al. 2022 showed the effect of non-ideal conditions on lead-based CH3NH3PbI3 perovskite material [5]. Despite the higher conversion efficiency in the laboratory, it has several issues in terms of commercialization such as environment protection from the toxic lead (Pb) component, and stability under environment conditions due to the organic component used in the ABM3 structure. To address such issues, researchers considered using lead-free inorganic material as the absorbing material for the photovoltaic applications. Titanium (Ti)-based PSCs were introduced by Ju et al. 2018 with 1.0-1.8 eV tuneable band gap, and Chen et al. (2018) achieved 1.02 V open circuit voltage (VOC, V), 5.69 mA/cm 2 current density (JSC, mA/cm 2 ) and 56.4% fill factor (FF, %) with Cs2TiBr6 absorbing material [6,7]. Shyma et al. 2022 employed SCAPS-1D-based simulation on Sn-based perovskite material CH3NH3SnI3 to investigate the various parameters of the active materials and the selection of the appropriate ETL (electron transport layer) for the device [8]. On the other side, Omarova et al. 2022 showed the selection of the optimal HTL (hole transport layer), and also determined that electrodes can minimize the effects of material defects to improve the device performance [9].
By taking the knowledge from the above discussion, our research article has pinpointed several vital contributions including developing a theoretical model of a lead-free mixed halide FTO/CdS/Cs2TiI6−XBrX /CuSCN/Ag-based structure where all the materials used for the SCAPS simulation are inorganic in nature, and then important physical, optoelectronic parameters such as thickness of material, device temperature and defect density of the material are optimized. For this optimization, we have analyzed the device performance parameter PCE with the variation in thickness of the active materials, followed by the device performance with the device temperature and defect density.

Device Architecture and Simulation
In our proposed FTO/CdS/Cs2TiI6−XBrX/CuSCN/Ag-based planar solar cell device model, the band gap of ETL (electron transport layer) materials, CdS and HTL material CuSCN are taken to be 2.4 eV, 3.26 eV and 3.4 eV, respectively, and the absorbing layer is tuneable under 1.0-1.78 eV [10][11][12][13][14][15]. The working temperature for the simulation is maintained at 27 °C with −0.8 V to 0.8 V bias voltage in the SCAPS-1D simulator. Here, Figure  1 shows the schematic view of the proposed structure. the simulation is carried out with illumination of AM 1.5 with the light power of 1000 W/m 2 under Gaussian energy distribution, and its characteristic energy is set to 0.1 eV. With Br doping the lattice parameter, band structure and the optical properties of Cs2TiI6 can be changed. Thus, due to the doping effect, both Cs2TiI2Br4 and Cs2TiI3Br3 are suitable for solar cell applications. On the other side, based on the superior optical coincidence index and better absorption coefficient, Cs2TiI2Br4 and Cs2TiIBr5 are ideal for light harvesting applications [16]. The details of the device materials' properties and active materials' basic parameters taken for the work are shown in Table 1 and Table 2, respectively. The symbols, i.e., Eg, denote the band gap energy; ꭓ is the electron affinity; ℇr is the relative permittivity; NA is the acceptor density; ND is the donor density; Nt is the total defect density; NC is the conduction band effective density of states; NV is the valence band effective density of states, respectively. The default values of some parameters and settings are as follows: electron mobility is 4.4 cm 2 /Vs, hole mobility is 2.5 cm 2 /Vs and electron and hole thermal velocity is 10 7 cm/s [17]. The Shockley-Queisser limit of a perfect photovoltaic absorbing material is around 1.3 eV [16]. Our Cs2TiI6−XBrX-based active material has a band gap in the range of 1.07 to 1.78 eV which can be seen in Table 1.

is the electron affinity;
Micromachines 2023, 14, x FOR PEER REVIEW 2 of 10 3rd generation PV technology. Perovskite solar cells (PSCs) have a great capability towards photovoltaic applications and are rapidly emerging as 3rd generation solar cells [1,2]. The journey started with Kojima et al. 2009, with halide perovskite ABM3 (A: organic CH3NH3 + ; B: Pb, M: Br, I), and they recorded 3.8% of power conversion efficiency (PCE, %) [1]. Later, Yang et al. 2015, andYin et al. 2015 further carried out extensive research on material, deposition process, fabrication methods and device structure that enhanced the PCE up to 20.1% experimentally, and theoretically 31.4% [3,4]. Hosseini et al. 2022 showed the effect of non-ideal conditions on lead-based CH3NH3PbI3 perovskite material [5]. Despite the higher conversion efficiency in the laboratory, it has several issues in terms of commercialization such as environment protection from the toxic lead (Pb) component, and stability under environment conditions due to the organic component used in the ABM3 structure. To address such issues, researchers considered using lead-free inorganic material as the absorbing material for the photovoltaic applications. Titanium CH3NH3SnI3 to investigate the various parameters of the active materials and the selection of the appropriate ETL (electron transport layer) for the device [8]. On the other side, Omarova et al. 2022 showed the selection of the optimal HTL (hole transport layer), and also determined that electrodes can minimize the effects of material defects to improve the device performance [9]. By taking the knowledge from the above discussion, our research article has pinpointed several vital contributions including developing a theoretical model of a lead-free mixed halide FTO/CdS/Cs2TiI6−XBrX /CuSCN/Ag-based structure where all the materials used for the SCAPS simulation are inorganic in nature, and then important physical, optoelectronic parameters such as thickness of material, device temperature and defect density of the material are optimized. For this optimization, we have analyzed the device performance parameter PCE with the variation in thickness of the active materials, followed by the device performance with the device temperature and defect density.

Device Architecture and Simulation
In our proposed FTO/CdS/Cs2TiI6−XBrX/CuSCN/Ag-based planar solar cell device model, the band gap of ETL (electron transport layer) materials, CdS and HTL material CuSCN are taken to be 2.4 eV, 3.26 eV and 3.4 eV, respectively, and the absorbing layer is tuneable under 1.0-1.78 eV [10][11][12][13][14][15]. The working temperature for the simulation is maintained at 27 °C with −0.8 V to 0.8 V bias voltage in the SCAPS-1D simulator. Here, Figure  1 shows the schematic view of the proposed structure. the simulation is carried out with illumination of AM 1.5 with the light power of 1000 W/m 2 under Gaussian energy distribution, and its characteristic energy is set to 0.1 eV. With Br doping the lattice parameter, band structure and the optical properties of Cs2TiI6 can be changed. Thus, due to the doping effect, both Cs2TiI2Br4 and Cs2TiI3Br3 are suitable for solar cell applications. On the other side, based on the superior optical coincidence index and better absorption coefficient, Cs2TiI2Br4 and Cs2TiIBr5 are ideal for light harvesting applications [16]. The details of the device materials' properties and active materials' basic parameters taken for the work are shown in Table 1 and Table 2, respectively. The symbols, i.e., Eg, denote the band gap energy; ꭓ is the electron affinity; ℇr is the relative permittivity; NA is the acceptor density; ND is the donor density; Nt is the total defect density; NC is the conduction band effective density of states; NV is the valence band effective density of states, respectively. The default values of some parameters and settings are as follows: electron mobility is 4.4 cm 2 /Vs, hole mobility is 2.5 cm 2 /Vs and electron and hole thermal velocity is 10 7 cm/s [17]. The Shockley-Queisser limit of a perfect photovoltaic absorbing material is around 1.3 eV [16]. Our Cs2TiI6−XBrX-based active material has a band gap in the range of 1.07 to 1.78 eV which can be seen in Table 1.
is the relative permittivity; N A is the acceptor density; N D is the donor density; N t is the total defect density; N C is the conduction band effective density of states; N V is the valence band effective density of states, respectively. The default values of some parameters and settings are as follows: electron mobility is 4.4 cm 2 /V s , hole mobility is 2.5 cm 2 /V s and electron and hole thermal velocity is 10 7 cm/s [17]. The Shockley-Queisser limit of a perfect photovoltaic absorbing material is around 1.3 eV [16]. Our Cs 2 TiI 6−X Br X -based active material has a band gap in the range of 1.07 to 1.78 eV which can be seen in Table 1.     [6,7]. Shyma et al. 2022 employed SCAPS-1D-based simulation on Sn-based perovskite material CH3NH3SnI3 to investigate the various parameters of the active materials and the selection of the appropriate ETL (electron transport layer) for the device [8]. On the other side, Omarova et al. 2022 showed the selection of the optimal HTL (hole transport layer), and also determined that electrodes can minimize the effects of material defects to improve the device performance [9]. By taking the knowledge from the above discussion, our research article has pinpointed several vital contributions including developing a theoretical model of a lead-free mixed halide FTO/CdS/Cs2TiI6−XBrX /CuSCN/Ag-based structure where all the materials used for the SCAPS simulation are inorganic in nature, and then important physical, optoelectronic parameters such as thickness of material, device temperature and defect density of the material are optimized. For this optimization, we have analyzed the device performance parameter PCE with the variation in thickness of the active materials, followed by the device performance with the device temperature and defect density.

Device Architecture and Simulation
In our proposed FTO/CdS/Cs2TiI6−XBrX/CuSCN/Ag-based planar solar cell device model, the band gap of ETL (electron transport layer) materials, CdS and HTL material CuSCN are taken to be 2.4 eV, 3.26 eV and 3.4 eV, respectively, and the absorbing layer is tuneable under 1.0-1.78 eV [10][11][12][13][14][15]. The working temperature for the simulation is maintained at 27 °C with −0.8 V to 0.8 V bias voltage in the SCAPS-1D simulator. Here, Figure  1 shows the schematic view of the proposed structure. the simulation is carried out with illumination of AM 1.5 with the light power of 1000 W/m 2 under Gaussian energy distribution, and its characteristic energy is set to 0.1 eV. With Br doping the lattice parameter, band structure and the optical properties of Cs2TiI6 can be changed. Thus, due to the doping effect, both Cs2TiI2Br4 and Cs2TiI3Br3 are suitable for solar cell applications. On the other side, based on the superior optical coincidence index and better absorption coefficient, Cs2TiI2Br4 and Cs2TiIBr5 are ideal for light harvesting applications [16]. The details of the device materials' properties and active materials' basic parameters taken for the work are shown in Table 1 and Table 2, respectively. The symbols, i.e., Eg, denote the band gap energy; ꭓ is the electron affinity; ℇr is the relative permittivity; NA is the acceptor density; ND is the donor density; Nt is the total defect density; NC is the conduction band effective density of states; NV is the valence band effective density of states, respectively. The default values of some parameters and settings are as follows: electron mobility is 4.4 cm 2 /Vs, hole mobility is 2.5 cm 2 /Vs and electron and hole thermal velocity is 10 7 cm/s [17]. The Shockley-Queisser limit of a perfect photovoltaic absorbing material is around 1.3 eV [16]. Our Cs2TiI6−XBrX-based active material has a band gap in the range of 1.07 to 1.78 eV which can be seen in Table 1.  ; B: Pb, M: Br, I), and they recorded 3.8% of power conversion efficiency (PCE, ) [1]. Later, Yang et al. 2015, andYin et al. 2015 further carried out extensive research on aterial, deposition process, fabrication methods and device structure that enhanced the CE up to 20.1% experimentally, and theoretically 31.4% [3,4]. Hosseini et al. 2022 showed he effect of non-ideal conditions on lead-based CH3NH3PbI3 perovskite material [5]. Depite the higher conversion efficiency in the laboratory, it has several issues in terms of ommercialization such as environment protection from the toxic lead (Pb) component, nd stability under environment conditions due to the organic component used in the BM3 structure. To address such issues, researchers considered using lead-free inorganic aterial as the absorbing material for the photovoltaic applications. Titanium ) and 56.4% fill factor (FF, %) with Cs2TiBr6 absorbing material [6,7]. Shyma et al. 022 employed SCAPS-1D-based simulation on Sn-based perovskite material H3NH3SnI3 to investigate the various parameters of the active materials and the selection f the appropriate ETL (electron transport layer) for the device [8]. On the other side, marova et al. 2022 showed the selection of the optimal HTL (hole transport layer), and lso determined that electrodes can minimize the effects of material defects to improve he device performance [9].
By taking the knowledge from the above discussion, our research article has pinointed several vital contributions including developing a theoretical model of a lead-free ixed halide FTO/CdS/Cs2TiI6−XBrX /CuSCN/Ag-based structure where all the materials sed for the SCAPS simulation are inorganic in nature, and then important physical, optolectronic parameters such as thickness of material, device temperature and defect density f the material are optimized. For this optimization, we have analyzed the device perforance parameter PCE with the variation in thickness of the active materials, followed by he device performance with the device temperature and defect density.

. Device Architecture and Simulation
In our proposed FTO/CdS/Cs2TiI6−XBrX/CuSCN/Ag-based planar solar cell device odel, the band gap of ETL (electron transport layer) materials, CdS and HTL material uSCN are taken to be 2.4 eV, 3.26 eV and 3.4 eV, respectively, and the absorbing layer is uneable under 1.0-1.78 eV [10][11][12][13][14][15]. The working temperature for the simulation is mainained at 27 °C with −0.8 V to 0.8 V bias voltage in the SCAPS-1D simulator. Here, Figure shows the schematic view of the proposed structure. the simulation is carried out with lumination of AM 1.5 with the light power of 1000 W/m 2 under Gaussian energy distriution, and its characteristic energy is set to 0.1 eV. With Br doping the lattice parameter, and structure and the optical properties of Cs2TiI6 can be changed. Thus, due to the dopg effect, both Cs2TiI2Br4 and Cs2TiI3Br3 are suitable for solar cell applications. On the ther side, based on the superior optical coincidence index and better absorption coeffiient, Cs2TiI2Br4 and Cs2TiIBr5 are ideal for light harvesting applications [16]. The details f the device materials' properties and active materials' basic parameters taken for the ork are shown in Table 1 and Table 2, respectively. The symbols, i.e., Eg, denote the band ap energy; ꭓ is the electron affinity; ℇr is the relative permittivity; NA is the acceptor denity; ND is the donor density; Nt is the total defect density; NC is the conduction band efective density of states; NV is the valence band effective density of states, respectively. he default values of some parameters and settings are as follows: electron mobility is 4.4 m 2 /Vs, hole mobility is 2.5 cm 2 /Vs and electron and hole thermal velocity is 10 7 cm/s [17]. he Shockley-Queisser limit of a perfect photovoltaic absorbing material is around 1.3 eV 16]. Our Cs2TiI6−XBrX-based active material has a band gap in the range of 1.07 to 1.78 eV hich can be seen in Table 1.

Optimization of Absorbing Layer Thickness with CdS Layer
In this section, the optimization of thickness of different perovskite absorbing layers such as Cs 2 TiBr 6 , Cs 2 TiI 1 Br 5 , Cs 2 TiI 2 Br 4 , Cs 2 TiI 3 Br 3 , Cs 2 TiI 4 Br 2 , Cs 2 TiI 5 Br 1 and Cs 2 TiI 6 has been studied at 27 • C temperature with standard defect density (~1014 cm −3 ) and a constant series resistance [18] by varying the thickness from 0.3 to 3.0 µm. Here, Figure 2a-c represent the PCE; a J-V graph with the thickness variation as 0.3 µm, 0.4 µm, 0.5 µm, 1.0 µm, 1.5 µm, 2.0 µm, 2.5 µm, 3.0 µm, 3.5 µm and 4.0 µm for the Cs 2 Ti-6−X Br X perovskite solar cell; and PCE with the back contacts' metal work function. Generally, the lower thickness of absorbing material leads to a low absorption of sunlight, which has a direct impact on PCE. From Figure 2a,b, it is observed that after 0.5 µm thickness for the Cs 2 TiBr 6 , Cs 2 TiI 1 Br 5 , Cs 2 TiI 2 Br 4 , Cs 2 TiI 3 Br 3 and Cs 2 TiI 4 Br 2 , and after 1.0 µm thickness for the Cs 2 TiI 5 Br 1 and 0.4 µm thickness for the Cs 2 TiI 6 absorbing layer, the PCE of the device cell starts to fall gradually. The reason behind this fall is due to the increment in thickness of the absorbing layer as the light absorption rate becomes much higher. Such excess light absorption leads to a temperature rise in the device, and thus V OC starts to fall drastically due to this temperature rise [12]. With the drop in V OC , the PCE of the device starts to decrease. We all are aware that the PCE is a key parameter to evaluate the total annual power generation of a perovskite solar cell device-based PV (photovoltaic) module. Thus, reducing the PCE will decrease the maximum generated power (P MAX ) in the device. Despite the V OC starting to fall with thickness, the generation rate of the charge carrier increases, which has a direct impact on the enhancement of J SC [19]. Such an enhancement in J SC will increase the Shockley-Read-Hall (SRH) recombination [20,21]. This phenomenon could establish a V OC -J SC relationship in the PV module [20][21][22].
where T = device temperature, I 0 = reverse saturation current, q = electronic charge, n = ideality factor and K = Boltzmann constant. From Equation (1), it is shown that the V OC starts to fall as the reverse saturation current (I 0 ) increases in the device. Therefore, by analyzing all the parameters we may conclude that the Cs 2 TiBr 6 , Cs 2 TiI 1 Br 5 , Cs 2 TiI 2 Br 4 , Cs 2 TiI 3 Br 3 and Cs 2 TiI 4 Br 2 materials are optimized at 0.5 µm; the Cs 2 TiI 5 Br 1 material is optimized at 1.0 µm; and the Cs 2 TiI 6 material is optimized at 0.4 µm.
From Figure 2c it is observed that the PCE increases with the higher metal work function of the Ag paste up to a certain work function value (5 eV). The reason behind such an increment in the work function value is the decrement of the height of the carrier barrier, and as a result, the metal contact becomes ohmic in nature [8]. Thus, open circuit voltages also increase.
On the other hand, Figure 3a,b indicate the changes in diffusion length (L n ) with the voltage variation at the ambient temperature (300 K). As per the observations, the diffusion length increases with the open circuit voltage (V OC ) for all the seven absorbing materials except for Cs 2 TiI 5 Br 1 and Cs 2 TiI 6 , where the diffusion length decreases after 0.6 V. The concentrations of electrons and holes are enhanced with the voltage which leads to an increment in diffusion length. The diffusion length of the electrons and holes should be higher than the absorbing layer thickness [8], since Cs 2 TiBr 6 , Cs 2 TiI 1 Br 5 and Cs 2 TiI 6 absorbing materials have a band gap above 1.50 eV which requires large energy to excite an electron in the conduction band. Thus, such materials operate in a higher temperature which can burn the device after a certain temperature. Therefore, despite the better power absorption and power generation capability of Cs 2 TiBr 6 , Cs 2 TiI 1 Br 5 and Cs 2 TiI 6 absorbing material, they are not suitable for PV application.
From Equation (1), it is shown that the VOC starts to fall as the reverse saturation current (I0) increases in the device. Therefore, by analyzing all the parameters we may conclude that the Cs2TiBr6, Cs2TiI1Br5, Cs2TiI2Br4, Cs2TiI3Br3 and Cs2TiI4Br2 materials are optimized at 0.5 µm; the Cs2TiI5Br1 material is optimized at 1.0 µm; and the Cs2TiI6 material is optimized at 0.4 µm. From Figure 2c it is observed that the PCE increases with the higher metal work function of the Ag paste up to a certain work function value (5 eV). The reason behind such an increment in the work function value is the decrement of the height of the carrier barrier, and as a result, the metal contact becomes ohmic in nature [8]. Thus, open circuit voltages also increase.
On the other hand, Figure 3a,b indicate the changes in diffusion length (Ln) with the voltage variation at the ambient temperature (300 K). As per the observations, the diffusion length increases with the open circuit voltage (VOC) for all the seven absorbing mate- ing materials have a band gap above 1.50 eV which requires large energy electron in the conduction band. Thus, such materials operate in a higher which can burn the device after a certain temperature. Therefore, despite the absorption and power generation capability of Cs2TiBr6, Cs2TiI1Br5 and Cs2T material, they are not suitable for PV application.
From Equation (2) we can conclude that as the temperature starts to open circuit voltage (VOC) will decrease accordingly. The prime reason behin From Equation (2) we can conclude that as the temperature starts to increase, the open circuit voltage (V OC ) will decrease accordingly. The prime reason behind such a decrement in V OC is the exponential inverse increment in r 0 if in Equation (2), which leads to a similar exponential inverse increment in I 0 due to the temperature rise [12]. On the other hand, the temperature increment may cause the increment in the recombination process which will lead to the increment in the short circuit current (I SC ) [21]. Figure 4a,b show the changes in the PCE and V OC with the device temperature for the absorbing materials and suggest that up to 40 • C, the V OC increases quite significantly for the Cs 2 TiBr 6 , Cs 2 TiI 1 Br 5 and Cs 2 TiI 2 Br 4 absorbing material, and after that temperature there is a significant change observed in the V OC . Such a higher V OC starts to increase the P MAX . The reasons behind such peculiarities are the higher band gap of the Cs 2 TiBr 6 (1.78 eV), Cs 2 TiI 1 Br 5 (1.58 eV) and Cs 2 TiI 2 Br 4 (1.38 eV) perovskite layers, and as the temperature starts to increase initially, the band gap of the materials starts to reduce, leading to a higher increment in the V OC for the Cs 2 TiBr 6 (1.78 eV) and Cs 2 TiI 1 Br 5 (1.58 eV) material. However, for the Cs 2 TiI 2 Br 4 material after 40 • C, V OC starts to drop. On the other hand, Cs 2 TiI 3 Br 3 , Cs 2 TiI 4 Br 2 , Cs 2 TiI 5 Br 1 and Cs 2 TiI 6 materials have a lower band gap. So, when the temperature starts to rise, the open circuit voltage (V OC ) starts to drop after 20 • C onwards despite the existence of almost constant J SC . As a result, PCE and P MAX after 20 • C start to reduce with the voltage drop. The same will be evidenced from Equation (2). This means as the temperature starts to rise, V OC starts to decrease continuously and the back recombination process is begun. As a result, the PCE starts to fall gradually. So, from the above analysis, we may conclude that the optimal temperature for the Cs 2 TiBr 6 , Cs 2 TiI 1 Br 5 and Cs 2 TiI 2 Br 4 materials is 40 • C and for the Cs 2 TiI 3 Br 3 , Cs 2 TiI 4 Br 2 , Cs 2 TiI 5 Br 1 and Cs 2 TiI 6 absorbing layers is 20 • C.  Similarly, changes in electron and hole diffusion length are depicted at 0 °C to working temperature in Figure 5a,b. It can be observed that at a relatively higher ature, the value of Ln is increasing simultaneously, as the diffusion length depend operating temperature and dopant concentration. Similarly, changes in electron and hole diffusion length are depicted at 0 • C to 100 • C working temperature in Figure 5a,b. It can be observed that at a relatively higher temperature, the value of L n is increasing simultaneously, as the diffusion length depends on the operating temperature and dopant concentration. Similarly, changes in electron and hole diffusion length are depicted at 0 °C to working temperature in Figure 5a,b. It can be observed that at a relatively higher t ature, the value of Ln is increasing simultaneously, as the diffusion length depends operating temperature and dopant concentration.

Optimization of Defect Density
The total defect density plays a key role in determining the performance of a skite solar cell device, as it can debase the performance quality of the device. It has heavy charge recombination between the interfaces [25]. In our present study, w changed the total defect density (Nt, cm −3 ) from 10 10 to 10 20 in the perovskite ab layer with a cell architecture of FTO/CdS/Cs2Ti-6−XBrX/CuSCN to understand its ef the device performances (PCE), and then tried to find out the optimum defect den all the materials at the optimized thickness and temperature. It is observed that at a total defect density, there is a higher recombination of electron and hole pairs du generation of pinholes at the electrodes. Such a phenomenon reduces the stabilit device and overall performance of the device. The effects of defect density at the d interfaces are not included in this study. Here, Figure 6 shows the PCE variatio defect density for the Cs2Ti-6−XBrX absorbing layer with the CdS electron transpor respectively. So, from the above figures, it is clearly observed that the increment in density above 10 10 cm −3 will reduce the PCE, respectively, for all the perovskite ma We can also observe that for the Cs2TiBr6 absorbing material, the PCE decreases cantly with the defect density as it has a higher band gap as a result of the recomb

Optimization of Defect Density
The total defect density plays a key role in determining the performance of a perovskite solar cell device, as it can debase the performance quality of the device. It has caused heavy charge recombination between the interfaces [25]. In our present study, we have changed the total defect density (N t , cm −3 ) from 10 10 to 10 20 in the perovskite absorbing layer with a cell architecture of FTO/CdS/Cs 2 Ti-6−X Br X /CuSCN to understand its effects on the device performances (PCE), and then tried to find out the optimum defect density for all the materials at the optimized thickness and temperature. It is observed that at a higher total defect density, there is a higher recombination of electron and hole pairs due to the generation of pinholes at the electrodes. Such a phenomenon reduces the stability of the device and overall performance of the device. The effects of defect density at the different interfaces are not included in this study. Here, Figure 6 shows the PCE variation with defect density for the Cs 2 Ti-6−X Br X absorbing layer with the CdS electron transport layer, respectively. So, from the above figures, it is clearly observed that the increment in defect density above 10 10 cm −3 will reduce the PCE, respectively, for all the perovskite materials. We can also observe that for the Cs 2 TiBr 6 absorbing material, the PCE decreases significantly with the defect density as it has a higher band gap as a result of the recombination rate starting to reduce. As a result, the collection of electron-hole pairs is also reduced and J SC starts to decrease. Such changes result in the reduction of V OC . The Cs 2 TiI 1 Br 5 , Cs 2 TiI 2 Br 4 , Cs 2 TiI 3 Br 3 , Cs 2 TiI 4 Br 2 , Cs 2 TiI 5 Br 1 and Cs 2 TiI 6 materials have a continuous V OC drop with J SC reduction, and as a result the PCE falls. This incident signifies that at the higher total defect density, the back recombination process increases due to the impurities' enhancement in the absorbing materials, and as a result, conversion efficiency (PCE) loss starts to increase due to the V OC and J SC losses [24]. Such losses in the parameters will decrease the P MAX for the absorbing layers which can be observed in Figure 6. So, from the above analysis of the performance parameters, the optimal defect density for the C-2 TiI 6−X Br X absorbing layer with CdS ETL is 10 10 cm −3 . Hence, it can be concluded that the total defect density of the materials affects the PCE of the device as increasing defects imply reduction in the diffusion length of the electron and hole charge carriers.
Micromachines 2023, 14, x FOR PEER REVIEW Figure 6. PCE of C-2TiI6−XBrX absorbing layer with different defect densities at optimized and temperature for CdS ETL.

Conclusions
This research article presents the optimization of the thickness of the absorb with two different electron transport layers (CdS) numerically using the SCAPS s for the planar FTO/CdS/Cs2TiI6−XBrX /CuSCN structure. The results indicate the mance of the device (PCE, VOC) and maximum power conversion (PMAX) are better 0.5 and 1 µm. Through further observations, we have seen that optimization of th temperature lies between 10 °C and 60 °C and defect densities between 10 10 and for the different absorbing materials. During the simulation process, it was obser defect densities have a great impact on the charge recombination rate. This resear has not covered the recombination rate in different interfaces of the solar cel which include the ETL/perovskite and perovskite/HTL interfaces.

Conclusions
This research article presents the optimization of the thickness of the absorbing layer with two different electron transport layers (CdS) numerically using the SCAPS simulator for the planar FTO/CdS/Cs 2 TiI 6−X Br X /CuSCN structure. The results indicate the performance of the device (PCE, V OC ) and maximum power conversion (P MAX ) are better between 0.5 and 1 µm. Through further observations, we have seen that optimization of the device temperature lies between 10 • C and 60 • C and defect densities between 10 10 and 10 14 cm −3 for the different absorbing materials. During the simulation process, it was observed that defect densities have a great impact on the charge recombination rate. This research work has not covered the recombination rate in different interfaces of the solar cell device, which include the ETL/perovskite and perovskite/HTL interfaces.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this research are available on request from the corresponding author.